I’ve been watching videos on YT about astronomy for years now. It’s fascinating. I never really ask questions and just believe what the experts have to say. But today I have decided to ask this question… A question that I’ve often ignored before.
So yesterday, I was watching this video and it mentioned a star/some celestial body. I don’t remember exactly, sorry. It said that the astronomers have calculated that this celestial body is about *13 billion lightyears* away from us and this many billions-something-huge.
It’s just so baffling to me. How do they do it? They’re calculating masses and distances of bodies that are supposedly soo sooo huge and far away, when we haven’t even actually managed to step on mars, yet. My point is, how are we capable of determining all this accurately when on a universal scale, we’re capable of pretty much nothing. How do these calculations work?
In: Physics
**Parallax**
First, we need a point of reference and the best reference point we have is the Sun. While ancient astronomers attempt to determine the distance to the Sun, they lacked the appropriate tools to make precise enough measurements, or knowledge of orbital mechanics to determine this with any reasonable degree of accuracy.
By the 17th century, we had a much better understanding of the orbits of planets, and were able to make connections about the orbits of the planets relative to each other. All we needed now was to know one of them.
The “one” was Venus. Occasionally, Venus will make a “transit.” That is, it’ll pass between the Earth and the Sun in a way that is visible. By measuring how long this transit lasts, we were able to, in the 18th century, calculate Venus’ orbit and, hence, Earth’s orbit, including our distance to the Sun.
Second, now that we know the distance to the Sun, we can use something called parallax to measure the distance from Earth to other stars. What you do is pick a star and measure the angle from Earth to that star. Then you wait 6 months and measure the angle to that star again. These two angles, plus the known distance between the position of the Earth (6 months apart, on opposite sides of the sun), uniquely defines a triangle whose apex is the chosen star. Once you know those three pieces of information, you can derive all the other information about that star, including the distance to it.
However, the further away something is, the more accurate you have to measure to get a good distance. Given our current tools, parallax only really works for things up to 100 light years away from Earth.
**Apparent Luminosity**
In the early 1900’s we discovered a type of star known as a Cepheid star. The cool thing about them is they dim and brighten in regular intervals. The interval depends only on the stars absolute brightness. A thing about luminosity is, the further away something is, the dimmer it appears. So if you look at one of these Cepheid stars and measure the period of its intervals, you can determine how bright it really is. If you compare how bright it really is with how bright it appears, you can determine how far away it is (since the rate at which things appear dimmer is mathematically related to how far away it is).
Cepheid stars are fairly common and fairly bright over all, allowing us to measure distances up to millions of light-years away.
We can use similar methods for other phenomenon with known brightnesses, such as supernovae, to measure distances even further away, up to a billion light years.
**Red Shift**
Lastly, is that the universe is expanding. The further two objects are away from each other, the faster they are moving away. When light is emitted from an object moving away from you, this “stretches out” the light which makes it appear “redder” than it really is^(*). So if you look at a kind of object whose brightness is known, you can measure how red-shifted its light is which tells you how fast it is moving away from you. Since the speed at which the object is moving away from you is related to its distance from you, you can then calculate its distance.
* – Not necessarily *literally* redder (though possibly so) but rather its wavelengths are longer (having been stretched out). This is in contrast to objects moving towards each other, whose wavelengths are squashed up and therefore smaller which makes it bluer. The terms red and blue refer to the fact that red and blue occupy the long and short wavelengths of our visible light spectrum, respectively.
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